1. Field of the Invention
The present invention relates to an image forming apparatus which is compact and operates at high speed, and an image forming process.
2. Description of the Related Art
In recent years, image forming apparatuses allowing for achievement of high image quality of 1,200 dpi or more have had two major problems to solve. One is a demand for achievement of high-speed performance, and the other is a demand for achievement of compactness.
For the former, in order to improve productivity in image forming apparatuses, improvement in printing speed is vital. As for a monochrome machine, measures are generally taken by increasing the linear velocity of a photoconductor (hereinafter possibly referred to as “electrophotographic photoconductor”, “latent electrostatic image-bearing member”, “image-bearing member” or “photoconductive insulator”) and enlarging the diameter of the photoconductor. As for a full-color machine, there are two directions, one is achievement of tandem technologies (a plurality of image forming elements are used), and the other is the direction in which measures are taken by increasing the linear velocity and enlarging the diameter of a photoconductor, as in the case of a monochrome machine. Here, the image forming elements denote a minimum unit structure for image forming, including at least a photoconductor, a charging member, a writing member and a developing member. In addition, a transfer member and a fixing member, a cleaning member, a charge-eliminating member, etc. may be provided; however, when a plurality of image forming elements are used at the same time, what is necessary is not a plurality of these image forming units but one unit formed in a combined manner.
Meanwhile, methods for forming multicolor images and full-color images are, in general, broadly divided into two methods using an image forming apparatus based upon an electrophotographic system. Specifically, they are image forming apparatuses based upon a “tandem system” in which image forming units are provided for each color, and based upon a “one-drum system”. An image forming apparatus based upon a “tandem system”, which is the former, produces a large number of printed sheets per unit time but has problems with a large size of the apparatus and its high costs because image-forming processors such as a charger and a laser scanner unit are necessary for each image forming unit; whereas, an image forming apparatus based upon a “one-drum system”, which is the latter, makes it not necessary to improve positional accuracy as high as that of the “tandem system” because displacement of an output image caused by using a plurality of photoconductor drums is vanishingly small in comparison with the “tandem system”, thereby making it possible to reduce costs caused by using four photoconductor drums. Also, the “one-drum system” is advantageous in that it is possible to make an image forming apparatus compact at the same time; therefore, note has been taken of it in recent years.
Additionally, as to a full-color image forming apparatus of the “one-drum system”, as shown in FIG. 9 (although FIG. 9 is basically for explaining an image forming apparatus of the present invention, a full-color image forming technique of a conventional “one-drum system” will be explained here for the sake of convenience, with reference to this figure), the following method is also described, for example in Japanese Patent Application Laid-Open (JP-A) No. 3-192282: toner images of each color formed by developing devices of each color (4Y), (4M), (4C) and (4K) in a developing unit (4) on a photoconductor drum (1) charged by a charger (2) and selectively exposed by an exposer (3) are not sequentially transferred onto a recording material (11) but once primarily transferred onto an intermediate transfer belt (5) by means of the electric field of a transfer roller (transfer member) (10); the toner images of four colors transferred onto this intermediate transfer belt (5) are transferred onto the recording material (11) at one time by means of the electric field of a secondary transfer roller (6); and then the unfixed toner image is fixed. Note that in the figure, the reference numeral (40) denotes a rotor which rotates with the developing devices mounted thereupon, (6) denotes a secondary transfer roller, (8) denotes a cleaning device which cleans the surface of the photoconductor drum (1), and (9) denotes an intermediate transfer cleaning device which cleans the surface of the intermediate transfer belt (5).
Giving greater freedom with respect to the placement of each device in an image forming apparatus than the case of the use of a transfer drum in the “one-drum system” and a method of holding and conveying a recording material on a conveyance belt to conduct transfers in the “tandem system”, the foregoing method using the intermediate transfer belt (5) has been suitably used in recent years in terms of the ability of making image forming apparatuses compact and suitability for a wide variety of recording materials, and has become the mainstream of color image forming apparatuses.
Incidentally, in the case of the “one-drum system”, toner images of four colors are formed using one photoconductor drum, so that even if there is an alteration in the rotation of the photoconductor drum, bringing the formation positions of the images of each color in line on the photoconductor drum makes the effect of the rotational alteration of the photoconductor drum appear in a similar manner in the toner images of each color; accordingly, the “one-drum system” is characterized in that by bringing the image forming positions of each color in line on the photoconductor drum, changes in hue rarely occur even when nonuniformity of image density attributable to rotational alteration of the photoconductor drum arises. Also, amongst processors disposed in the vicinity of one photoconductor drum, anything except a developing unit allows the same thing to be used for each color; therefore, there is even such a characteristic that it is possible to simplify the structure of the apparatus and it is possible to make the apparatus compact and to lower costs. However, in the “one-drum method”, there is a problem that it takes approximately four times longer for the “one-drum system” to form a full-color image by means of four colors of yellow, magenta, cyan and black than to obtain a monochrome image of black color, and thus the productivity in producing full-color images per unit time (printing speed) is low.
Since it has the merits and demerits, the image forming system according to the “one-drum system” is employed in full-color machines aimed at serving also to produce black-and-white images, as things stand at present.
However, in conventional image forming apparatuses, since members constituting image forming elements such as for a charging step and a writing step are slow in ability, it has been difficult to plan compactness, high-speed performance (50 sheets/min or more) and high resolution (1,200 dpi or more).
In a charging step, it is necessary to improve charging ability for achievement of high-speed performance. When the diameter of a photoconductor is lessened, the width by which a charging member and the photoconductor can be disposed so as to face each other (referred to as charging nip) becomes very small. It is not impossible for a wire-type charging member used thus far, which is typified by a scorotron charger, to increase the amount of a corona falling onto the surface of the photoconductor by increasing the number of wires, but there is a problem that when wires are too close to one another in distance, they interfere with one another and power consumption becomes greater. Additionally, a grid is necessary for stabilizing charging, and the size thereof determines the charging nip width. A grid is generally made of a conductive metal plate and placed in the tangential direction of a photoconductor. For this reason, when the diameter of a photoconductor is lessened, a grid-photoconductor surface distance significantly differs between the center and both ends of a grid, and the net nip width becomes very small (charging becomes unstable at both ends that are front and rear ends corresponding to the moving direction of the photoconductor). In order to solve this problem, it is possible to use a grid which is not flat to fit the curvature of a photoconductor. However, an apparatus has to be a little complex to place such a photoconductor in, and the space in which a charging member can be placed is inevitably small due to the reduction in diameter; thus, this method is not realistic.
In contrast to the method, there is a method in which a roller-shaped charging member is used. A roller-shaped charging member is used in such a manner as to make contact with a photoconductor surface, or in such a manner that both surfaces thereof are placed close to each other with a gap of 50 μm or so in between. In most situations, by rotating both surfaces (making both surfaces rotate together) at equal speed, and applying a bias voltage to the roller, a discharge takes place from the roller to the photoconductor, and the photoconductor surface is charged. In this case, the charging member can successfully be made compact by lessening the diameter of the roller to a possible extent. When the roller diameter is lessened, the chargeable range (such a range that the photoconductor is apart from the roller surface by roughly 50-100 μm; referred to as charging nip) becomes narrow, and thus charging ability is lowered. However, it is not lowered as much as that of the scorotron charging does; further, charging ability improves dramatically, as a bias voltage applied to a roller member includes not only a DC component but also an AC component in a superimposed manner. By using such a technique, a charging step is no longer a rate-limiting factor in an image forming process at present. However, owing to the AC superimposition to obtain greater charging ability, there is a greater hazard to the photoconductor surface, and so the impact on the durability (lifetime) of the photoconductor will be great.
Meanwhile, in a writing step, light-emitting diodes (LEDs) and laser diodes (LDs) have been used as writing light sources until these days. LEDs are used as a writing light source, as they are placed in the form of an array in the lengthwise direction of a photoconductor (in the case of a drum-shaped small-diameter photoconductor, not in the MD (machine direction) but in the TD (traverse direction), in other words the axial direction), in the vicinity of the photoconductor. However, its resolution is determined by the size of one element, and also depends upon the distance between elements. Therefore, at this point in time, an LED is hardly deemed to be most suitable as a light source of 1200 dpi or more. Meanwhile, when an LD is used, writing is carried out by drawing and sending a light beam in the lengthwise direction of a photoconductor by means of a polygon mirror. When the diameter of a photoconductor is lessened, photoconductor linear velocity increases in relation to printing speed, and thus there is a need to increase the number of rotations of the polygon mirror. However, at present the number of rotations of a polygon mirror is 40,000 rpm or so at the most, and a single beam causes a limit on writing speed.
In contrast to the foregoing, a system in which a plurality of light beams are used has come into use. The following are used: a system of irradiating one polygon mirror with beams from a plurality of LD light sources; and a multi-beam exposing unit such as a construction in which a plurality of LDs are disposed as one array. Also these days, as multi-beam units, a surface-emitting laser with three light sources or more is used, and further, a surface-emitting laser with its light source placed in a two-dimensional manner is used. These techniques have been making it possible to carry out writing on photoconductors with a resolution of 1200 dpi or more.
As just described, amelioration of members constituting image forming elements or a novel technique has been making compactness, high-speed performance (50 sheets/min or more) and high resolution (1200 dpi or more) of photoconductors ready to achieve.
Meanwhile, in actual fact, as to related art such as the one described above, when compactness and high-speed performance are to be realized at the same time, it is not much clear where a rate-limiting factor is, in process designing, owing to the relationships between the linear velocity of a photoconductor, the size of members disposed in the vicinity of the photoconductor, and their respective abilities; furthermore, photoconductor techniques to respond to demands for compactness and high-speed performance have yet to become clear.
Therefore, the present invention is aimed at solving the problems in related art, and achieving the following objects.
An object of the present invention is to provide a compact image forming apparatus capable of forming high-quality images at high speed, and an image forming process using the image forming apparatus. Also, an object is to provide an image forming apparatus which is high in durability and capable of stable image output with few abnormal images, even when repeatedly used, and an image forming process using the image forming apparatus.